| As a rapidly developing short-distance optical wireless communication technology,the visible light communication(VLC)based on white LED has the advantages of large capacity,high bandwidth,anti electromagnetic interface,high security,free spectrum application,and green energy saving etc.VLC can not only provide illumination for indoor,but also has tremendous potential for providing the user with high-speed data transmission services,which has attracted the attention and research of many scholars and enterprises.However,if VLC is used as a single wireless communication pattern,there are many shortcomings such as the communication link is easily blocked,the uplink is difficult to implement,and the modulation bandwidth of the LED is limited etc.Therefore,more and more researchers try to combine VLC with other wireless technologies to build indoor hybrid networks.Currently,in VLC-based indoor hybrid networks,the key technologies such as channel modeling,full-duplex communication,hybrid networking,hybrid convergence mechanisms,optimal network selection,multi-user access and handover mechanisms all need to break through.But how to combine the application requirement of the user with the performance of the network to select an optimal network in multiple candidate networks and how to reasonably use various channel resources to achieve multi-user access in hybrid networks are important issue that needs to be considered in implementing the VLC systems.First of all,the research background and significance of VLC-based indoor hybrid networks are introduced in this paper,followed by its research status around the world.Then,the basic principle and topology structure of indoor VLC system are introduced.Then the common network selection methods in hybrid networks are introduced,and the multiple access technologies are classified and introduced.In order to realize multi-user indoor VLC system,a VLC-based indoor hybrid network model is proposed,in which the Wi Fi and Infrared(IR)channels are used for uplink,and VLC and Wi Fi channels are used for downlink.In order to effectively combine the subjective application requirement of the user with the objective performance of the network for network selection,an optimal network selection method based on multiple attribute decision making(MADM)is proposed.Wherein,the analytic hierarchy process(AHP)method is used to obtain the subjective weight of the decision parameter,and the standard deviation method is used to obtain the objective weight of the decision parameter,and a minimization mathematical model is used to obtain the partition coefficients of the subjective and objective weight,and then the simple addictive weighting(SAW)method is used to get the evaluation value of each candidate network.In addition,an improved beacon frame structure is proposed to ensure real-time access selection.Through simulation analysis,the proposed network selection method can satisfy the application requirement of the user,and in accordance with the objective network performance.In order to ensure multi-user effectively access the hybrid network,based on the CSMA/CA protocol,an VH(Virtual Handover)-based multi-user access method is proposed in this paper,which comprehensively considers different channel and MAC information.In our multiuser access method,the concept of backoff lock is proposed to control the access request of the user to different channel,and the concept of VH is proposed to reduce the access delay and improve the access success ratio.When the determinant conditions of VH are satisfied,the user can use the backoff lock to lock the current access request,and reselect an optimal or smaller delay network to send access request packet.Moreover,the expressions of collision probability,access delay and access success ratio in indoor hybrid network are given.Through simulation analysis,the proposed multi-user access method is applicable and feasible for VLC-based indoor hybrid networks,and the method can reduce the collision probability and the access delay,and improve the access success ratio. |
